Most modern motherboard chipsets provide a USB host controller. Older machines
which are not equipped with a USB host controller can be upgraded using a PCI
cards with a host controller built in.

USB host controllers are
compatible with either the Open Host Controller Interface (OHCI, by Compaq) or
the Universal Host Controller Interface (UHCI, by Intel) standard. Both types
have the same capabilities and USB devices work with both host controller
types. Basically the hardware of UHCI is simpler and therefore it is
cheaper, but needs a more complex device driver, which could cause slightly
more CPU load.